It is known, in particular from the international patent applications WO-A-2007/099246 and WO-A-2005/085727, to enrich the air of the blast of a blast furnace with oxygen using oxygen generated by a unit for the separation of the air gases (Air Separation Unit or ASU).
It is also known to use a top gas recovery blast furnace, hereinafter denoted by the abbreviation TGRBF.
The TGRBF makes it possible to reduce the consumption of coke in the blast furnace. This technology is also one of the favored solutions envisaged by the iron and steel industry to reduce the CO2 emissions of blast furnaces by capture and/or value enhancement and/or sequestration of CO2 generated by blast furnaces.
In the literature, the following designations are found for this type of blast furnace:                top gas recovery blast furnace or TGRBF,        ULCOS (Ultra Low CO2 Steelmaking) blast furnace,        oxygen blast furnace, and        nitrogen-free blast furnace.        
The principle on which a TGRBF is based is that of separating the top gases resulting from the blast furnace into at least a fraction enriched in CO2 and a fraction enriched in CO. The fraction enriched in CO2 is captured and sequestered, used on site or, preferably, enhanced in value. The fraction enriched in CO is re-injected into the TGRBF.
For efficient operation of a TGRBF, the top gases generated by the blast furnace have to consist predominantly of carbon oxides (CO and CO2). To this end, instead of air, a gas richer in oxygen is used for the blast injected into the TGRBF.
A specific example of such a known TGRBF is described in DE-A-2037541. According to this process, a portion of the recovered top gas is injected mid-stack as a reducing gas. A second portion of the recovered top gas is mixed with oxygen and this mixture is injected as blast into the lower part or base of the blast furnace. In order to achieve temperatures analogous to those achieved with a blast consisting of air, it is proposed to use a mixture of recovered top gas and oxygen having an oxygen content identical to that of air. It is also possible to envisage using mixtures exhibiting other oxygen contents, given the reduced consumption of coke in the TGRBF and the corresponding reduction in the need for blast for the combustion of the coke in the base of the blast furnace.
It is possible in particular to provide an air separation unit located close to the TGRBF. Such an air separation unit receives compressed air, supplied by an air compressor or a blower, and separates this compressed air into at least an oxygen-rich gas and a nitrogen-rich gas. The oxygen-rich air gas is then used as blast in the TGRBF.
In the present context “oxygen-rich” gas is understood to mean a gas having an oxygen content greater than the oxygen content of air (that is to say, greater than 21.0 vol % O2 and up to 100.0 vol % O2) and a “nitrogen-rich” gas is understood to mean a gas having a nitrogen content greater than the nitrogen content of air (that is to say, greater than 78.1 vol % N2 and up to 100.0 vol % N2).
At least (a) a blast comprising oxygen-rich gas supplied by the air separation unit and (b) top gas enriched in recovered CO2 are thus continuously injected into the TGRBF.
The arrangement necessary for the operation of conventional blast furnace plants comprises the blast furnace as such, one or more air blowers for the blast, chimneys, and the like, and requires major capital costs.
A top gas recovery blast furnace plant furthermore requires an air separation unit in order to supply an oxygen-rich gas and a plant for the treatment of top gas and for the recovery of the fraction enriched in CO and, preferably, also a plant for the conditioning of the fraction enriched in CO2.
What is more, given that the TGRBF top gas is generated by the TGRBF itself, the operator of the TGRBF does not have available, at the beginning of a cold start-up of the TGRBF, a (sufficient) stream of top gas enriched in CO to be recovered and injected into the blast furnace.
Consequently, the operator of the TGRBF has to take into account the absence of this reducing gas at the time of the cold start-up of the TGRBF.
Thus, the operator can provide an initial interim stage before the continuous operation of the TGRBF during which a blast poorer in oxygen is injected into the TGRBF, which blast consists of compressed air, indeed even of compressed air enriched in oxygen. Said compressed air for the start-up of the blast furnace is provided by an additional compressor or blower, which requires a not insignificant additional capital cost.